Alloys resistant to high temperatures



United States Patent A LI .OYS RESISTANT TO HIGH TEMPERATURES William T.Kaarlela, Fort Worth, Tex., assignor to General Dynamics Corporation,Fort Worth, Tex., a corporation of Delaware No Drawing. Originalapplication Apr. 9, 1963, Ser. No. 271,577, now Patent No. 3,220,828,dated Nov. 30, 1965. Divided and this application Apr. 15, 1965, Ser.

3 Claims. (Cl. 75134) This application is a division of applicantscopending application Serial No. 271,577, filed April 9, 1963, nowPatent No. 3,220,828.

This invention relates in general to refractory metal alloys capable ofresisting high temperatures and more particularly, to alloys of suchcharacter as may be well suited for the brazing of refractory metalssuch as columbium, tantalum, molybdenum, and their alloys, and havingadditional utility as high temperature protective coatings and asstructural alloys.

It has become increasingly important particularly in missile, spacecraftand aircraft applications, to use materials which are capable ofwithstanding extremely high temperatures. In addition to molybdenum andtantalum, one such material is columbium, which is a refractory metalpossessing a melting point in the vicinity of 4400 F. From a designstandpoint, it is an excellent material because of its high strength toweight ratio in the 2000 to 2500 F. range of service temperatures. Itdoes not strain-harden rapidly; allowing cold Working up to 99% withoutannealing, and it is therefore particularly suitable for the forming ofparts of complex shape. Columbium is also characterized by moderatedensity (comparable to iron and nickel), and a high melting point, withgood strength retention above the useful range of currently availablealloys.

Useful though it is in high temperature areas, joinder' of the metal andits alloys present problems. Thus, although it may be welded, nitrogencontamination is a deleterious obstacle since it causes an increase inthe tendency for crater cracking, serious loss of ductility and anincrease in the transition temperature. Welding also presents theproblem of loss of strength due to recrystallization. In the handling ofhigh strength columbium alloys, recrystallization occurs between 2200and 2800 F.,'depending upon the alloy makeup. Welding involves thesehigh temperatures and where recrystallization as a result occurs, a lossof approximately 50% in tensile strength may be anticipated.

However, it has been found that by using the alloy of this invention,excellent joinder of the metals is effected and a high temperatureresistant joint is produced which is compatible with the strengthcharacteristics of the joined materials. Vacuum environment or inertatmospheres such as argon gas are utilized to prevent oxidation of thecolumbium and its alloys during the brazing; only moderate efforts beingnecessary for purification of the argon gas preparatory to brazing.

The alloy of this invention is additionally useful as a coating forcolumbium and for other materials characterized by low oxidationresistance at elevated temperatures, i.e., of approximately 2000 F.

Further utility for this alloy is found in structural applications, suchas castings, and in other areas of service where adequate oxidationresistance at elevated temperatures is difiicult to achieve and maintainand where both structural and non-structural provisions are arequirement.

Accordingly, it is an object of this invention to provide an alloy wellsuited for the brazing of refractory metals such as columbium,molybdenum, tantalum, and their 3,262,773 Patented July 26, 1966 alloys;which alloy is capable of providing excellent joint strength at elevatedtemperatures.

It is another object of this invention to provide an alloy of thecharacter described which does not require excessively high temperaturesfor brazing and which possesses adequate ductility.

A further object is to provide an alloy, as described, which does notcause excessive erosion of the base alloys when applied thereto inbrazing applications.

Yet a further object is to provide an alloy suitable as a protectivecoating for preventing oxidation of materials having a highsusceptibility thereto at elevated temperatures.

Another object is the provision of an alloy adapted to castingapplications which calls for materials possessing adequate oxidationresistance .at elevated temperatures.

These and other objects and advantages of this invention Will becomeapparent from the following description of the alloy and itscharacteristics and the claims directed thereto.

In general the alloy of this invention includes titanium as a matrixelement in the percentage-by-weight ranges indicated. This matrixelement is characterized by its compatibility with columbium,molybdenum, tantalum, and the alloys thereof. The alloying elementsadded to the matrix element comprise chromium, columbium and germanium,and are proportioned as hereinafter set forth. The resulting alloy hasproduced brazed joints having joint strength at high temperatures. Thisbrazing has been effected without the use of extremely hightemperatures. The ability of the alloy to Withstand high temperatureswith a minimum of deterioration has contributed to its furtherutilization as a coating and as a barrier for protection againstoxidation of various materials inherently susceptible thereto. Usabletensile strength of this alloy at temperatures around 2000 F. furtherdictate its use in the area of structural material, particularly incasting applications.

It is to be understood that percentages used herein in bothspecification and claims, to describe ingredient proportions, arepercentages by weight unless otherwise stated.

Titanium as an elemental constituent of the alloy has been found to bequite compatible with the refractory metals, forming a tough, narrow,diffusion layer at the interface with the brazing alloy. As employedherein, the general range of titanium is from about 20% to about 90% byweight of the alloy. A preferred range has been found to be from 30% toabout Superior alloy compositions have been established incorporatingthe specific proportions of titanium tabulated below.

Chromium, like titanium, is quite compatible with the refractory metalsforming a dilfusion layer at the interface with the brazing alloy. Asemployed herein the general range for chromium is from about 20% toabout 65% by weight of the alloy. A preferred range has been found to be30% to about 48%. Superior alloy compositions include chromium in thequantities set forth hereinafter.

The addition of columbium improves the toughness and ductility of thealloy, and further finds use in adjusting the alloy melting temperatureand reducing the alloy erosion characteristics, while significantlyincreasing the high temperature strength. The general range forcolumbium is from about 5% to about 35%, and the preferred range is fromabout 10% to about 30%.

Germanium serves to improve oxidation resistance and depresses themelting point. For germanium, the general range is from about 5% toabout 25%, and the preferred range is from about 7% to about 16%.

Although the process for alloy formulation is subject to considerablevariation, the alloy of the present inven tion has, for test purposes,been formulated by mixing the elemental ingredients together in thedesired proportions in powder form. The mixture is subsequentlybriquetted into a compact; then melted in a cold-hearth, water-cooled,copper crucible. If ductile, the alloy is then rolled to form a foil, orin the alternative, broken and crushed into a powder to be used in thisform.

As formulated herein, the alloy of this invention has taken the powderform. Application is effected by mixing the powder alloy with polyvinylalcohol in a slurry, which is then painted on the joint to be brazed.The alloy is then heated to a temperature above its melting point in aprotective environment, such as in a vacuum or using an argon atmosphereor other suitable inert gases for protection against the detrimentaleffect of oxidation. As hereinabove stated, moderate efforts should bemade to purify the argon gas, if best results are to be obtained. Thishas been accomplished satisfactorily, in the present instance, bypassing the argon through a -100 F. Dry Ice-acetone cold trap, and aclosed zirconia tube filled With titanium strips and operating at 1750F. The protection offered by the inert argon gas is important. Shouldatmospheric contamination occur it will be directly reflected in reducedflow and wetting of the brazing alloy, resulting in an inferior joint.

For purposes of the tests, the results of which are reflected in thetabulations below,'brazed lap shear test specimens are made up usinga-columbium alloy incorporating by weight titanium and 10% molybdenum.These were employed as the members to be joined and lap shear tested.Time at temperature prior to testing was 1-5 minutes. Employing anA-frame type lap shear tension test apparatus, failure was made to occurwithin one minute by steadily increasing the mechanical stress upon thespecimen by means of a floating screw. Test temperatures were effectedby the induction heating of a graphite susceptor surrounding thespecimen. Stress was measured by means of a calibrated load link inconjunction with a strain recorder. Specimens were confined underprotective, substantially inert, argon atmosphere during heating,testing and cooling.

The alloy of this invention is set forth in the table below with anindication both as to the general range of its ingredients and as to thespecific composition of the Further, the alloy was tested for structuralintegrity, brazing characteristics and environmental resistance. Suchvalues as brazing temperature, re-melt temperature,

shear strength, toughness factor and brazeability on columbium,molybdenum and tantalum were established and are set forth in tabularform below:

During these tests, a remelt temperature rise phenomenon was observed inconnection With the alloy in the above tabulation of test results. Thatis to say the brazing temperature of the alloy was 2650 F. However, intests after brazing of the columbium T specimens, it was found that thealloy did not remelt up to a temperature of more than. 3500 F.

Thus, a highly useful titanium-chromium based alloy having columbium andgermanium for braze joining or coating of refractory metals,particularly where high temperature use is contemplated, has beendevised, yet one which permits brazing to be accomplished at atemperature which is not harmful to the structure, has a minimaloxidation and erosion characteristic on the joined parts, and achieveshigh temperature strength.

I claim:

1. An alloy characterized by its ability to withstand high temperaturesand consisting of from about 20% to about titanium, from about 20% toabout 65% chromium, from about 5% to about 35% columbium, and from about5% to about 25% germanium.

2. An alloy characterized by its ability to withstand high temperaturesand consisting of from about 30% to about 48% titanium, from about 30%to about 48% chromium, from about 10% to about 30% columbium, and fromabout 7% to about 16% germanium.

3. An alloy characterized by its ability to withstand high temperaturesand consisting of about 35 titanium, about 35 chromium, about 20%columbium, and about 10% germanium.

References Cited by the Examiner UNITED STATES PATENTS 2,169,193 8/1939Comstock 134.3 3,111,406 11/1963 Kaarlela 75175.5 3,131,059 4/ 1964Kaarlela 75-176 DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner.

1. AN ALLOY CHARACTERIZED BY ITS ABILITY TO WITHSTAND HIGH TEMPERATURESAND CONSISTING OF FROM ABOUT 20% TO ABOUT 65% TITANIUM, FROM ABOUT 20%TO OUT 65% CHROMIUM, FROM ABOUT 5% TO ABOUT 35% COLUMBIUM, AND FROMABOUT 5% TO ABOUT 25% GERMANIUM.